1. Field of the Invention
The present invention relates to the art of wet flue gas desulfurization, utilizing a calcium compound such as limestone as an absorbent, and more particularly to a method for controlling the oxidation of sulfurous acid in an absorbing liquid.
2. Description of the Related Art
As for a wet flue gas desulfurization process, the state of the art utilizes a so-called in-situ process which does not require an oxidation tower and which operates by blowing air into a tank positioned at the bottom of an absorption tower in order to oxidize an absorbent slurry (an absorbing liquid in a form of a suspension of an absorbent comprising limestone) that has absorbed sulfurous acid to thereby produce gypsum as a by-product. In this case, the main reactions that take place during the treatment are indicated by the following reactions (1) through (3). ##EQU1##
In the case of the above-described in-situ process, it is particularly important to maintain the concentration of sulfurous acid in the absorbent slurry within a low level range close to zero (about 0 to 5 mmol/liter) by instantly oxidizing the sulfurous acid substantially completely to promote the reactions (2) and (3) without allowing the sulfurous acid derived from the reaction (1) to deposit as calcium sulfite.
In the case where oxidation is insufficient, such that the concentration of sulfurous acid rises, and particularly where calcium sulfite is deposited, some of the problems encountered will result in a significant drop in the desulfurization due to the inhibition of the reaction (1); the reactivity of the lime stone (calcium compound) drops; and the purity of gypsum (by-product) obtained by the reaction (3) decreases.
As a usual measure to avoid the above-mentioned problems, fine bubbles of oxidizing air (oxygen-containing gas) is continuously blown into the slurry of an absorbing tank to forcibly cause the oxidation reaction (2) to proceed. However, an excessive supply of air results in a rise in system operation cost.
Moreover, according to the recent studies of the present inventors, peroxides which have been generated by an excessive oxidation of slurry are found to deteriorate an adsorbent resin in the waste water treatment facility of the slurry so that the capacity to control a chemical oxygen demand (COD) deceases. On the other hand, if the concentration of sulfurous acid becomes higher because of an oxidation deficiency (insufficient oxidation), the COD value in waste water from the slurry is known to become higher.
Accordingly, also from the COD treating aspect, it is necessary to control the supply of oxidizing air to keep a flow rate within a minimal requirement and to maintain the concentration of sulfurous acid within the above-mentioned low level range with high reliability. Such methods for control include those disclosed by Japanese Patent Provisional Publication (JP-A) No.60-226,403 and Japanese Patent Provisional Publication (JP-A) No.61-433 (Japanese Patent Publication (JP-B) No.3-59,731).
Japanese Patent Provisional Publication (JP-A) No.60-226,403 discloses a process comprising continuous detection of a sulfite concentration (including the sulfurous acid present as an ion) in an absorbing liquid, and regulation of the oxidizing catalyst supply in accordance with a signal of deviation of the detected value from a preset value of the sulfite concentration. In this case, the sulfite concentration is determined by a procedure comprising taking a sample liquid by means of a measuring pump, generating a gas from the sample liquid by adding thereto an acid such as hydrochloric acid, and calculating the sulfite concentration of the sample liquid based on the concentration of sulfur dioxide in the generated gas of the preceding step.
Japanese Patent Publication (JP-B) No.3-59,731 discloses a process comprising the steps of continuously detecting an oxidation-reduction potential (ORP) of an absorbent slurry and effecting a feed back control so that the detected value equals a desired sulfite concentration to thereby keep calcium sulfite completely oxidized (i.e., within a low concentration range close to zero).
The conventional process disclosed in Japanese Patent Provisional Publication (JP-A) No.60-226,403 requires as long as 10 to 20 minutes for the measurement of the concentration of sulfite. Accordingly, this process is feasible in the case where the operational conditions including the quantity of sulfur dioxide [(flow rate of flue gas).times.(concentration of sulfur dioxide)] in the flue gas gradually change. However, a problem arises in the case where operational conditions abruptly change. For example, a load change of a boiler, which is a source of flue gas, is generally abrupt. Therefore, this process can hardly follow the fluctuation in an operational condition that accompanies the above-mentioned load change, thus has not been practicable.
On the other hand, according to the method for controlling ORP as disclosed in Japanese Patent Publication (JP-B) No.3-59,731, only a very short time is required for the detection of an ORP value and the detected ORP value sharply reflects the change in the concentration of sulfurous acid. Therefore, this process provides a practicable follow-up and controllability to some extent by a simple proportional control. For the realization of an oxidation control with a higher efficiency and reliability, however, the following disadvantages of the above-mentioned process need to be overcome.
(a) According to detailed analysis by the present inventors of the actual correlation between the concentration of sulfurous acid and ORP in an absorbent liquid in such a flue gas desulfurization apparatus, the correlation is expressed, for example, by an exponential curve as shown in FIG. 4 and reveals that the correlation significantly drops when OPR is on the decrease. Therefore, in a simple proportional control (or PID control) where a proportional gain is constant, an action to increase the flow rate of oxidizing air, at the time when a load change causes ORP to decrease (when the concentration of sulfurous acid increases), is delayed, results in an increased possibility that the concentration of sulfurous acid exceeds a proper range over a long period of time.
(b) In the case of an excessively oxidized state where a detected ORP value is well over a target value, if the flow rate of oxidizing air is reduced based on the deviation without any restriction, an action to increase the flow rate of oxidizing air to meet the case where a load change reverses the state to cause ORP to decrease abruptly is delayed, resulting in an increased possibility that the concentration of sulfurous acid exceeds a proper range over a long period of time.
(c) Since the correlation between ORP and the concentration of sulfurous acid in an absorbent liquid varies depending on pH values of the absorbent liquid, an action in a simple proportional control where the target value of ORP is constant, can take the concentration of sulfurous acid out of a proper range as a result of a change in pH of the absorbent liquid, despite the fact that ORP is controlled within a proper range.
(d) Since the change in the properties of an absorbent liquid, for example, due to contamination with impurities can abnormally change the correlation between ORP and the concentration of sulfurous acid in an absorbent liquid, and since the quantity of the oxidizing air basically required for the quantity of sulfur dioxide in the flue gas can vary, the action by the above simple proportional control may possibly take the concentration of sulfurous acid out of a proper range.
(e) The change in the quantity of the circulating absorbent liquid can cause the amount of auto-oxidation to vary, and may change the minimally required quantity of oxidizing air. Also in this case, an action in a simple proportional control may possibly take the concentration of sulfurous acid out of a proper range, and possibly further followed by an excessive supply of air.